The Reststrahlen effect (German: “residual rays”) is a reflectance phenomenon in which electromagnetic radiation within a narrow energy band cannot propagate within a given medium due to a change in refractive index concurrent with the specific absorbance band of the medium in question; this narrow energy band is termed the Reststrahlen band.

As a result of this inability to propagate, normally-incident Reststrahlen band radiation experiences strong-reflection or total-reflection from that medium.

The energies at which Reststrahlen bands occur vary and are particular to the individual compound.

Numerous physical attributes of a compound will have an effect on the appearance of the Reststrahlen band these include but are not limited to; phonon band-gap, particle/grain size, strongly absorbing compounds, compounds with optically opaque bands in the infrared.